A typical fuel supply device of an internal combustion engine mounted in a vehicle includes a fuel pump. In the fuel supply device, fuel is pressurized and a mist is sprayed via an injector. In recent years, a common-rail type internal combustion engine having a common rail for storing fuel to be supplied to the injector that has been pressurized in advance is widely known. In this type of internal combustion engine, the fuel is forced from the fuel pump to the common rail. Such fuel supply devices are disclosed Laid-Open Japanese Patent Application Publication No. 9-222056 and Laid-Open Japanese Patent Application Publication No. 2000-18052.
The fuel pump includes a pressure chamber. The volume of the pressure chamber is expanded and contracted by the power of the engine. Fuel having a relatively low pressure is introduced into the pressure chamber. The fuel is discharged during a contraction process in which the volume of the pressure chamber is contracted. The fuel is sealingly stored in the pressure chamber by closing an opening/closing valve at a predetermined time. Then, with the contraction of the volume of the pressure chamber, the fuel in the pressure chamber is forced out. Usually, a plurality of pressure chambers is provided, each of which is expanded and contracted at different times from other pressure chambers. A control unit controls the opening/closing valve so that the closing time is set in accordance with the fed quantity of fuel that is required. Putting the closing time ahead so as to seal the fuel in the pressure chamber when the volume of the pressure chamber is large increases the fed quantity of fuel. On the other hand, delaying the closing time so as to seal the fuel in the pressure chamber when the volume of the pressure chamber is small reduces the fed quantity of fuel. For example, in the aforementioned common-rail type, the pressure of the fuel in the common rail is detected and then the fed quantity of fuel that is required is set in such a manner that the detected pressure is a target pressure.
A cam that operates by the power of the engine achieves expansion and contraction of the volume of the pressure chamber. The contraction rate of the volume of the pressure chamber depends on the shape of the cam. In general, the contraction rate has a peak in the latter half of the contraction process in which the volume of the pressure chamber is contracted.